1. Field of the Invention
The present invention relates to an electro-photographic photoreceptor used for image formation in an electrophotographic system and to an image formation device provided with the photoreceptor.
2. Description of Related Art
An electrophotographic system image formation device (hereinafter also referred to as “electrophotographic device”) using electrophotographic technologies to form an image is used for many copying machines, printers, and facsimile devices.
In an electrophotographic device, an image is formed through the following electrophotographic processes.
First, the photoreceptor layer of the electrophotographic photoreceptor (hereinafter also referred to as “photoreceptor”) mounted on the device is made to charge uniformly to a given potential by a charger and then exposed to light such as laser light applied corresponding to image information from the exposure device to form an electrostatic latent image.
A developer is supplied to the formed electrostatic latent image from the developing device to stick colored microparticles called a toner which is a component of the developer to a surface of the photoreceptor to develop the electrostatic latent image, thereby visualizing the latent image as a toner image.
The formed toner image is transferred to a transfer material such as recording paper from a surface of the photoreceptor by the transfer device and then fixed by the fixing device to form a desired image.
In the transfer action of the transfer device, the toner on a surface of the photoreceptor is not fully transferred to the transfer material but a part of the toner is left on the surface of the photoreceptor. Further, there is the case where a paper powder of recording paper remains stuck to the surface of the photoreceptor.
Foreign substances such as these residual toners and stuck paper powder adversely affect on the quality of a formed image and are therefore removed by a cleaning device.
With the recent development of cleaner-less technologies, a developing means into which a cleaning function is incorporated without independent cleaning devices, that is, a system having both developing and cleaning functions is used to recover residual toners and to remove foreign substances such as a stuck paper powder.
After a surface of the photoreceptor is cleaned, a charge of a surface of the photoreceptor is removed by a charge removing device to make the electrostatic latent image disappear.
The photoreceptor used in this electrophotographic process is constituted by laminating a photoreceptor layer containing a photoconductive material on a conductive substrate.
The photoreceptor material is largely divided into an inorganic photoconductive material and an organic photoconductive material.
The inorganic photoconductive material has recently come to be scarcely used as a photosensitive material because of its toxicity. However, a non-pollutant amorphous silicon type (a-Si) photoreceptor is still being developed.
Though the a-Si photoreceptor has merits such as high sensitivity and high durability, it has a drawback that it is difficult to form the photosensitive layer uniformly, so that image defects are easily caused. Also, the a-Si photoreceptor has drawbacks including low productivity and high production cost.
Since the inorganic type photoreceptors have many drawbacks as mentioned above, the development of photoconductive materials used to form the photoreceptor are forwarded and many organic type photoconductive materials, that is, organic photoconductors (abbreviation: OPC) have come to be largely used.
Though electrophotographic photoreceptors using organic type photoconductor materials (hereinafter referred to also as an “organic photoreceptor”), have some problems concerning sensitivity, durability and stability to environments, they have more advantages than inorganic photoreceptors in the points of toxicity, production cost and degree of freedom in design of materials.
The organic photoreceptor also has the advantage that the photosensitive layer constituting the photoreceptor can be formed by known easy and economic methods represented by a dip coating method.
The organic photoreceptor has many advantages as mentioned above, and therefore has gradually come to occupy the mainstream of the photoreceptor.
Also, along with recent studies and development, the sensitivity and durability of the organic photoreceptor have been improved and therefore, the organic photoreceptor has come to be used except for special cases.
In particular, the performance of the organic type photoreceptor has been significantly improved with the development of the function separation type photoreceptor containing different materials assigned to have a charge generation function and charge transportation function separately.
Specifically, the function separation type photoreceptor has a further advantage that the material constituting the photosensitive layer can be selected from a wide range of materials and therefore, a photoreceptor having desired characteristics can be produced relatively easily, besides the above advantages that the organic type photoreceptor has.
In electrophotographic devices, the above charge, exposure, developing, transfer, cleaning and charge-removal actions are practically exerted on the photoreceptor repeatedly under various environments. Therefore, it is demanded of the photoreceptor to have high environmental stability, electrical stability and durability (printing durability) against mechanical external force besides high sensitivity and high responsibility to light.
Specifically, the photoreceptor is desired to have high printing durability so that the surface layer thereof is resistant to abrasion caused by the sliding contact with the cleaning member.
To take appropriate measures to improve the printing durability, an attempt is made to add filler particles in the charge transport layer of a laminate type photoreceptor to thereby improve the printing durability. However, there is the possibility of image defects caused by nonuniformity of a layer in the vicinity of the boundary between the charge generation layer and the charge transport layer which is considered to be due to the interaction between the filler particles and the charge generation layer, showing that the effect of the attempt is not said to be sufficient.
Moreover, when a filler is added to the charge transport layer, this gives rise to the production of a trap with a size extending to tens of micrometers over the entire charge transport layer between filler particles and a polymer bulk (binder resin) contained in the photoreceptor, which remarkably increases the risk of a rise in the residual potential of the exposure part.
In light of this, technologies in which a surface protective layer on the outermost layer of a photoreceptor (see, for example, Japanese Patent Application Laid-Open No, 57-30846), technology in which lubricity is provided to the surface protective layer (see, for example, JP-A No. 64-23259), technologies in which the surface protective layer is hardened (see, for example, JP-A No. 61-72256) and technologies in which the surface protective layer is made to contain filter particles (see, for example, JP-A No. 1-172970).
Among the above technologies, the technologies in which the surface protective layer is made to contain filler particles involves such a new factor as the control of the dispersibility of particles, which has an effect on characteristics of the photoreceptor.
Specifically, the characteristics of the photoreceptor are not defined only by simple addition of fillers. It is reported that the printing durability of the photoreceptor is improved by addition of a filler in an amount of about 0.1 to about 10% by weight based on the total solid of the surface protective layer (see, for example, JP-A No. 1-205171).
However, it is estimated with ease that a difference in the dispersed state of filler particles brings about a difference in the image characteristics/electric properties/printing durability of the photoreceptor as a photoreceptor drum.
Also, when the dielectric constant of the surface protective layer is non-uniform, there is the case where this causes a thick image to be formed at the edge part when a black solid image is output and a toner is scattered. It is found from this fact that the dispersion state of filler particles inside of the surface protective layer has a large influence on the characteristics of the photoreceptor.
Moreover, the addition of fillers with the intention of improving the printing durability gives rise to the problem described below. The problem is that the photoreceptor is easily affected by ozone emitted from a corona discharge device and oxidizing gases such as nitrogen oxides. As a result, the photoreceptor gives rise to a reduction in charge potential, a rise in residual potential and a reduction in surface resistance, resulting in a deterioration in resolution, a significant deterioration in output image and short life of the photoreceptor.
For these phenomena, there are proposals concerning measures taken to evade a direct influence of gas on the photoreceptor by exhausting and displacing the gas around the corona discharge device and measures taken to prevent the deterioration of the photoreceptor by adding an antioxidant and a stabilizer to the surface protective layer containing filler particles.
However, when an antioxidant and a stabilizer are added in a small amount to the surface protective layer containing filler particles, this is sometimes causes of a rise in residual potential from the first and abrasion of the film.
When an antioxidant and a stabilizer are added in such an amount as to stand to repeated use for a long period of time, on the other hand, this causes a rise in residual potential from the first and an increase in the abrasion of the film.
In other words, the above prior technologies have not succeeded in developing an excellent photoreceptor having both printing durability and ozone resistance at the same time yet. Also, such practically unfavorable defects that the electrophotographic characteristics such as sensitivity and residual potential are impaired when an antioxidant is added as mentioned above still remain at present.
Therefore, useful proposals are expected as to a novel material which is improved in printing durability and ozone resistance and is entirely free from defects in electrophotographic characteristics.